It is known in the prior art to construct an abrasive propelling device using a pair of substantially parallel discs maintained in spaced relationship with a plurality of blades or vanes lying between the discs. Normally, abrasive material, commonly referred to as grit, is supplied to the center of the wheel while it is spinning and the material is caused to pass outwardly along the vanes to be thrown against a receiving surface at relative high velocity. A wheel of this general type is known from German Pat. No. 23 11 866 (which corresponds to U.S. Pat. No. 3,785,105) wherein the length of the radially extending centrifugal blades is shorter than the diameter of a central space circumscribed by the inner ends of the blades in their installed positions. A disadvantage of this structure is that it is necessary that the central aperture in the front wheel disc as well as the aperture in a cover surrounding the driving wheel be larger in their diameters than the total length of each centrifugal blade in order that the blade can be mounted by passing them through the central opening when it is necessary to replace the blades.
The outside diameters of the wheel discs depend upon the lengths of the centrifugal blades as a result of which no optimal design of the wheel is possible with regard to its centrifugal case in the case of simultaneously small rotating masses. An increase of the speed of release, under conditions of a constant speed, by the use of longer centrifugal blades is thus likewise not possible in this previously known wheel structure. The spacers between the wheel discs simultaneously perform the function of spacing and also act as stops for the restraint of the centrifugal blades against outward motion. Thus, it is necessary in this structure that a spacer must be assigned to each centrifugal blade. As a result of that, the spacers and thus also the front wheel disc which, in most cases, is weaker, are under unnecessary stress from the centrifugal force of the rotating centrifugal blades.
Another form of wheel is shown in German Pat. No. 21 15 354 (which corresponds to U.S. Pat. No. 3,745,711) which has centrifugal blades insertable from the outside to the inside. In a structure of this type, it is possible to insert centrifugal blades of any arbitrary length into a predetermined fly wheel structure, but in that case stops movable to the outside and in the form of rotatable bushings on the spacer bolts are needed for the support of the centrifugal blades. Such stops are very expensive and, moreover, it is often necessary to expend considerable energy to move them, and then only with special tools, because of particles of the blasting medium which become jammed in between relatively movable components when replacement of the blades is desired.